"PC users have volumes of information saved on their computers, most of it disconnected and disparate save for a basic directory system. The answer to connecting all the information into a local semantic Web of information is closer than you might think. Thanks to the open source NEPOMUK (Networked Environment for Personalized, Ontology-based Management of Unified Knowledge) effort, the Semantic Desktop isn't a dream; it's an emerging reality and will be here with the upcoming release of KDE 4 for the Linux desktop."

For some kind of data it might make sense to also store it in the file or in extended file system attributes. This is currently not implemented AFAIK and as Aaron pointed out, would be subject to policies.

IMHO all such metadata information should be stored per-file, ie. in extended attributes cos then it could be shared by all users. And I don't really like the idea of a single database with all the metadata cos it could get corrupted, or if it got deleted one way or another you'd lose ALL metadata. But to have public and private metadata saved per-file would most likely require modifications to existing filesystems, or a completely new one. Hopefully this will happen, I'm sure a lot of people would find such a thing useful even if I can't imagine myself gaining much from that.